Conventional Pneumatic Floating Fender
Conventional pneumatic floating fenders are typically used to absorb dynamic impact loading between a jetty and a ship side shell, and, in naval applications, between two ships to avoid direct shell-to-shell impact. Referring to FIG. 1A, a conventional pneumatic floating fender 100 is usually composed of one outer protection cover and two body layers, as follows:
(1) The outer protection cover consists of used tires 101, steel chains 102, a valve 103 and two towing rings 104 with one at each end. The purpose of this cover is to protect the rubber layer beneath against friction induced wearing damages. Impact loading induced friction forces acting at these tire 101 side surfaces should be transferred to the towing rings 104 at both ends of the fender 100 through these steel chains 102. The total weight of these tires and chains are very heavy, thus making transportation and offshore handling of the fender difficult;
(2) The first fender body layer is the outer rubber layer composed of rubber material and several layers of fiber nets such as polyester nets, which are bonded together through a vulcanization process to provide the structural strength of the fender. The same vulcanization process should also bond this layer to the steel surfaces of the two towing rings 104 to form a sealed room inside the fender 100;
(3) The second body fender layer is composed of a thin layer of rubber or synthetic materials for the purpose of air sealing, so that the pneumatic fender 100 can maintain its designed air pressure with a need for very few air pressure adjustments throughout its entire service life.
When a floating fender is placed between two ships side-by-side under offshore conditions, direct high impact forces acting at these tire side surfaces and lateral relative motions between the two ships could occur at the same time to produce a large lateral friction force at these loading surfaces. Such large friction force will provide little help for the reduction of a ship's lateral motion, as its magnitude is way too small compared with the wave induced movement of the ship. The magnitude of the friction force is a direct product of the impact force magnitude at the tire side surfaces and the friction coefficient of these surfaces. Because the friction coefficient of these tire side surfaces usually is significant (friction coefficient >1.0 or more at most tire side surfaces), and so the friction force is large enough to cause serious wearing damages to the tire side surfaces. This motion induced large friction force at tire side surfaces should also produce the same magnitude friction forces acting in the opposite direction at the ship outer shell surfaces to cause scratching damages to the ship shell surface paints. Furthermore, modern tires are usually equipped with steel wire nets bonded with the layers of rubber materials, which should make the scratching induced paint damages much worse. As the tire side surface wearing damage increases, steel chains, which pass through the holes at the middle height of each used tire, could directly contact ship outer surfaces to cause not only greater paint scratching damages, but also a potential structural damage to the ship outer shells as well. To the shipping industry as a whole, the pneumatic floating fender induced paint damages have become one of the major reasons for all ships to have to perform a scheduled re-painting work inside dry docks at required intervals. For navy ships, this protection of the outer shell paints is even more important than commercial ships, because some special paints can absorb radar waves to help make the ships stealthy.
Clearly there is a need in the shipping industry to reduce the paint damage induced costs associated with frequent scheduled re-painting work for each ship.
Foam filled fender is an alternative to standard pneumatic floating fenders for small sized vessels and warships. FIG. 1B is a conventional foam filled offshore fender 110 without an outer protection cover, in prior art. A foam filled fender 110 usually has an outer protection cover similar to the one of a pneumatic floating fender with used tires and steel chains. For the body of a foam filled fender 110, it usually has a rubber or polyurethane outer layer and foam 112 filled inner body. There is a steel chain 111 inside the fender to connect two towing rings 114 together at the two ends of the fender 110. The fender 110 performance properties can be modified by changing the grades of the foam 112. Without air pressure inside the fender 110, there should be no concerns for deflating, burst or sink. In addition, this configuration also eliminates the safety valves and ball valves, etc. with the reduction of associated maintenance costs of the fender 110 compared with a pneumatic floating fender 100. One disadvantage of a foam filled fender 110 is its big size during transportation and resultant high freight costs.
The best way to reduce the fiction force is to replace used tires and steel chains with a low friction coefficient material for the outer protection cover of a pneumatic floating fender. Nevertheless, this substitute material has to possess other required properties as well. The material selected in this disclosure is called Ultra High Molecular Weight Polyethylene (UHMWPE) plate, a widely used plastic material with some superior properties well suitable for the substitute material as the protection cover of a pneumatic floating fender. One typical UHMWPE plate (also called “plastic plate” in this document) has the following key properties:
(1) superior property to stand repeated impact loading and as well as to absorb impact loading energy with low noises;
(2) superior property to stand the friction induced wearing, e.g., an anti-wearing property 7 times better than that of a common carbon steel plate with a similar configuration, based on test data;
(3) a very low friction coefficient with a self-lubricant ability. Under offshore storm environments with wet plate surfaces, the measured friction coefficient is typically between 0.05˜0.1, which is about 1/10˜ 1/20 that of a tire side surface. In other words, the motion induced friction force by a UHMWPE plate could be as small as 5% of the one produced by a tire side surface under the same impact loading magnitude and the same lateral motions between two ships;
(4) superior property for not absorbing water when submerged, making it well suitable for offshore applications;
(5) density of 0.94 which is less than that of seawater, so capable of providing some buoyancy when submerged;
(6) One additional advantage of using plastic plates to replace conventional tires and steel chains is the ability to have different colors for the surfaces of these covers. In contrast, tires only have one fixed color of black, which is not an eye-catching color under offshore environments. The most visible color from a long distance away is orange, which is the common color for various safety wears on the sea. If plastic plates are used as the fender outer protection cover, a large selection of colors could be used including orange; and
(7) The unique property of the extra smooth surfaces of a UHMWPE plate makes it very difficult for marine growths to hang on to under offshore environmental conditions.
However, a UHMWPE plate has two properties which make it hard to be utilized to replace tires and steel chains as the outer protection cover for a pneumatic floating fender. First, the melting point is about 136° C. which is just below the temperature used for vulcanization of the fender body rubber layer. Second, UHMWPE is very difficult to be mixed or bonded with other materials such as a rubber layer. These two properties make the UHMWPE plate very difficult to be bonded directly with rubber layer through a vulcanization process. Therefore, the primary challenge for utilizing UHMWPE plates for the replacement of tires and chains is how to fix a UHMWPE flat and hard plate firmly to the surface of the fender body outer layer's curved and soft rubber surface. Secondly, the fixation should avoid producing excessive bending moment between the flat and hard plate and the curved and soft rubber surface due to fatigue concerns. At the same time, impact induced friction forces should be able to be transferred to the towing rings at the two ends of a pneumatic floating fender.